The present invention relates to the fabrication of cutting elements, particularly of the type in which a diamond covered compact is adhered to a stud.
One type of cutting element used in rotary drilling operations in earth formations is depicted in FIG. 1. That cutting element 8 comprises an abrasive composite or compact 10 mounted on a stud 12. The composite typically comprises a diamond layer 14 adhered to a cemented carbide substrate 16, e.g., tungsten carbide cemented by a metal binder such as cobalt. The substrate is brazed to the stud 12, and the stud is mounted in a drill bit by being fitted into predrilled holes in the drill bit.
During use of such a drill bit, it is not uncommon for the cutting elements to shear-off. In that regard, attention is directed to FIG. 1 herein in which is depicted the existence of fracture-susceptible zone 18 resulting from the geometry of the cutting element. That zone 18 is defined by a sharp corner formed by the intersection of the front surface 20 of the stud and the side surface 22 of the substrate 16. Thus, under the stresses induced during a cutting operation, a fracture line 24 can propogate rearwardly from the region 18, resulting in a shearing-off of the cutting portion of the cutting element.
One proposal for dealing with that problem involves the placement of a shroud 30 in supportive relationship to the compact 10 as depicted in FIGS. 2 and 3 and disclosed in greater detail in U.S. Pat. No. 4,632,196 issued Dec. 30, 1986 by the present inventor. That shroud includes an outer surface 31 which extends to, and is flush with, the peripheral surface 32 of the stud. The shroud bears against a peripheral edge portion of the compact for about 180 degrees to resist the cutting forces F which are applied to the compact during a cutting operation. In theory, it was hoped that the potential fracture point would be moved to the level 34 where the surface 20 intersects the peripheral surface 32 of the stud. Since the stud would be embedded sufficiently far into the bore in the drill bit, the drill bit would be reinforced around its periphery at that elevation 34 by the wall of the bore in which the stud is embedded. Therefore, the occurrence of fracturing would be resisted. In practice, however, there has occurred a tendency for fractures to form at higher elevations along the line of intersection of the surfaces 20 and 32, e.g., at elevation 36 in FIG. 2, as the cutting forces are transmitted to that region by the shroud 30. At that elevation, the stud is not reinforced by the bore wall, so fractures may occur.
It is, therefore, an object of the present invention to support a composite in such a manner that fractures do not tend to occur outside of a region of the stud which is to be reinforced by the bore wall.